Fix segfault when performing a second run with a reference after refe…

…rence structures have been cleared (#1041)

* Ensure the active reference is cleared when all references are cleared.

* Version 6.20.1. Revision bump for fixed segfault when running
'reference' after a clear.

src/DataSetList.cpp

@@ -1040,6 +1040,7 @@ void DataSetList::ClearRef() {
     for (DataListType::const_iterator ds = RefList_.begin(); ds != RefList_.end(); ++ds)
       delete *ds;
   RefList_.clear();
+  activeRef_ = 0;
   DataList_ = setsToKeep;
 }
 

src/Version.h

@@ -12,7 +12,7 @@
  * Whenever a number that precedes <revision> is incremented, all subsequent
  * numbers should be reset to 0.
  */
-#define CPPTRAJ_INTERNAL_VERSION "V6.20.0"
+#define CPPTRAJ_INTERNAL_VERSION "V6.20.1"
 /// PYTRAJ relies on this
 #define CPPTRAJ_VERSION_STRING CPPTRAJ_INTERNAL_VERSION
 #endif